Unit 1
Concept, Structure and Function of an Ecosystem

Learning Outcomes

Prerequisites

Key words

Ecosystem, Producers, Consumers, Decomposers, Food Web, Ecological Pyramid

Discussion

2.1.1 Concept, structure and function of an ecosystem

The interactions in nature are studied in a branch of science termed as ‘Ecology’. Do you know why the plants and animals in a pond differ from an ocean? Why is there a difference in forest types all over the globe?

All these questions find answers in the study of ecology and ecosystems. The term ‘Ecology’ was first coined by the German biologist Ernst Haeckel in 1869. Haeckel defined ecology as ‘the study of natural environment including the relations of organisms to one another and to their surroundings.’ It is derived from two Greek words – “oikos” meaning home and “logos” meaning study. Thus literally, ecology is the study of life at home with main emphasis on patterns of relations between organisms and their surrounding environment.

An ecosystem can be visualised as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment. The ecosystem is largely divided into two components, Abiotic and Biotic. Ecosystem structure is created due to interaction between abiotic and biotic components, varying over space and time.

2.1.1.1 Abiotic Components

The abiotic components of an ecosystem refer to the physical environment or the non-living factors. The organisms cannot live or survive without their abiotic components. They mainly include:

1. inorganic substances required by organisms such as carbon dioxide, water, nitrogen, calcium, phosphorus, etc. that are involved in material cycles. The amount of these inorganic substances present at any given time in eco-system is called as standing state or standing quality of ecosystem.
2. Organic compounds like proteins, carbohydrates, amino acids, lipids, humic substances and others are synthesized by the biotic counterpart of an ecosystem. They make the bio-chemical structure of the ecosystem.
3. Climatic factors including mainly rain, light, temperature, humidity, wind and air and
4. Edaphic and other factors such as minerals, soil, topography, pH, etc. greatly deter-mine the functions, distribution, structure, behaviour and inter-relationship of organisms in a habitat.

2.1.1.2 Biotic Components

The biotic components of the ecosystems are the living organisms including plants, animals and microorganisms. Based on their nutrition-al requirement, i.e. how they get their food, they are categorized into three groups:

1. Producers,
2. Consumers and
3. Decomposers

Producers

Producers are mainly the green plants with chlorophyll which gives them the ability to use solar energy to manufacture their own food using simple inorganic abiotic substances, through the process of photosynthesis. They are also called as photoautotrophs (pho-to = light, auto= self, troph = nutrition). This group is mainly constituted by green plants, herbs, shrubs, trees, phytoplanktons, algae, mosses, etc. This group of organisms uses so-lar energy (photosynthesis) or more rarely in-organic chemical reactions (chemosynthesis) to create food in the form of energy-rich molecules such as carbohydrates. This process, the synthesis of carbon dioxide into organic compounds, is called primary production and is directly or indirectly connected to the survival of all life on the Earth. Producers form the first link of a food chain. Since they are at the beginning of the food chain, producers are the direct or indirect source of food for other living organisms. There are some chemosynthetic bacteria (sulphur bacteria) beneath in the ocean which can synthesize their food in absence of sunlight, thus known as chemoautotrophs (chemo= chemical, auto= self, troph = nutrition).

Consumers

The next level of organisms that follows the producers are the consumers. Consumers are organisms that cannot prepare their own food and depend on plants and animals for food. Organisms that cannot make their own food are called heterotrophs, because they obtain food from other organisms rather than them-selves. Consumers lack chlorophyll, so they depend on producers for food. They are also known as heterotrophs. Consumers include mammals, birds, fish, reptiles, amphibians, insects, fungi and microscopic organisms such as protozoa and some types of bacteria. Depending on how they obtain food, there are four types of consumers: primary, secondary, tertiary and quaternary consumers. They mainly include herbivorous (feed on plants), carnivorous (feed on other animals), omnivorous (feed on both plants and animals) and detritivore organisms (feed on dead parts, waste, remains, etc. of plants and animals,).

Primary consumers: Feed directly on plants and other producers. Examples of primary consumers are zooplankton, butterflies, rab-bits, giraffes, pandas and elephants.
Secondary consumers: Feed on primary con-sumers, Examples of secondary consumers are earwigs, ants, badgers, snakes, rats, crabs, hedgehogs, blue whales (their diet is primar-ily composed of phytoplankton-eating krill and zooplankton, and phytoplankton), lions, and humans. Secondary consumers nearly always consume both producers and primary consumers and are therefore usually classed as omnivores.
Tertiary consumers: Feed on secondary con-sumers.
Quaternary consumers: A quaternary con-sumer is simply a consumer which preys upon a tertiary consumer. To be classed as a quaternary consumer within a food chain or food web, there must be a tertiary consumer available for the quaternary consumer to prey upon. Examples of quaternary consumers are the white shark, polar bear and alligator.

Decomposers

Decomposers (saprotrophs) are the micro-organisms, bacteria and fungi, which break down complex dead organic matter into simple inorganic forms, absorb some of the decomposition products, and release inorganic nutrients that are reused by the producers. All ecosystems have their own set of producers, consumers and decomposers which are specific to that ecosystem.

Decomposers or detrivores are a group of organisms consisting of small animals like worms, insects, bacteria and fungi, which break down dead organic material into smaller particles and finally into simpler substances that are used by plants as nutrition. Thus, decomposers are microorganisms that break down complex organic matter into simple inorganic matter through chemical reactions. All living organisms are made up of complex organic matter such as complex carbohydrates, proteins and fats. When they die, decomposers act on their dead bodies and return their organic matter back to nature in inorganic form. The inorganic matter enters the soil as nutrients that are absorbed by plants. Bacteria, protozoa and earthworms are examples of decomposers. Decomposers are an integral part of a food chain, as they convert organic waste materials into inorganic materials. De-composers complete a life cycle. They help in recycling the nutrients as they provide nutrients to soil or oceans, that can be utilised by autotrophs or producers, thus, starting a whole new food chain.

2.1.2 Concept and definition of Food Chain

The food chain is a linear sequence of organ-isms where nutrients and energy are transferred from one organism to the other. This occurs when one organism consumes another organism. It begins with producer organism, follows the chain and ends with decomposer organism. A food chain refers to the order of events in an ecosystem, where one living organism eats another organism, and later that organism is consumed by another larger organism. The flow of nutrients and energy from one organism to another at different trophic levels forms a food chain. The food chain also explains the feeding pattern or relationship between living organisms. Trophic level refers to the sequential stages in a food chain, starting with producers at the bottom, followed by primary, secondary and tertiary consumers. Every level in a food chain is known as a trophic level.

There are two types of food chains, namely detritus food chain and grazing food chain. Let’s look at them more closely:

1. Detritus food chain:
   The detritus food chain includes different species of organisms and plants like algae, bacteria, fungi, protozoa, mites, insects, worms and so on. The detritus food chain begins with dead organic material. The food energy passes into decomposers and detritivores, which are further eaten by smaller organisms like carnivores. Carnivores, like maggots, become a meal for bigger carnivores like frogs, snakes and so on. Primary consumers like fungi, bacteria, protozoans, and so on are detritivores which feed on detritus.
2. Grazing food chain:
   The grazing food chain is a type of food chain that starts with green plants, passes through herbivores and then to carnivores. In a grazing food chain, energy in the lowest trophic level is acquired from photosynthesis. In this type of food chain, the first energy transfer is from plants to herbivores.

This type of food chain depends on the flow of energy from autotrophs to herbivores. As autotrophs are the base for all ecosystems on Earth, the majority of ecosystems in the environment follow this kind of food chain.

2.1.3 Food Web

Several interconnected food chains form a food web. A food web is similar to a food chain but it comparatively larger than a food chain. Occasionally, a single organism is consumed by many predators or it consumes several other organisms. Due to this, many trophic levels get interconnected. The food chain fails to showcase the flow of energy in the right way. But the food web is able to show the proper representation of energy flow, as it displays the interactions between different organisms. When there are more cross interactions between different food chains, the food web gets more complex. This complexity in a food web leads to a more sustainable ecosystem. In a food chain, there is a singular path for energy flow and in a food web, there are different paths for energy flow.

2.1.4 Ecological Pyramid

An ecological pyramid is a graphical representation of the relationship between the different living organisms at different trophic levels. Ecological pyramids were first developed by Charles Elton. Since he first developed the ecological pyramid, it is also known as Eltonian pyramid. An ecological pyramid may be erect or inverted depending on the type of criterion and the ecosystem. It can be observed that these pyramids are in the shape of actual pyramids with the base being the broadest, which is covered by the lowest trophic level, i.e., producers. The next level is occupied by the primary consumers and so on.

2.1.4.1 Types of Ecological Pyramid

Three types of ecological pyramid exist. They are as follows,

1. Pyramid of Numbers

In this type of ecological pyramid, the number of organisms in each trophic level is considered as a level in the pyramid. The pyramid of numbers is usually upright except for some situations like that of the detritus food chain, where many organisms feed on one dead plant or animal. This shows the number of organ-isms in each trophic level without any consideration for their size. This type of pyramid can be convenient, as counting is often a simple task and can be done over the years to observe the changes in a particular ecosystem.

     b. Pyramid of Biomass

In this particular type of ecological pyramid, each level takes into account the amount of biomass produced by each trophic level. The pyramid of biomass is also upright except for that observed in oceans where large numbers of zooplanktons depend on a relatively smaller number of phytoplanktons. This indicates the total mass of organisms at each trophic level. Usually, this type of pyramid is largest at the bottom and gets smaller going up, but exceptions do exist. The biomass of one trophic level is calculated by multiplying the number of individuals in the trophic level by the average mass of one individual in a particular area. This type of ecological pyramid solves some problems of the pyramid of numbers, as it shows a more accurate representation of the amount of energy contained in each trophic level, but it has its own limitations. For example, the time of year when the data are gathered is very important, since different species have different breeding seasons.

c. Pyramid of Energy

Pyramid of energy is the only type of ecological pyramid, which is always upright as the energy flow in a food chain is always unidirectional. Also, with every increasing trophic level, some energy is lost into the environment. The pyramid of productivity looks at the total amount of energy present at each trophic level, as well as the loss of energy between trophic levels. Since this type of representation takes into account the fact that the majority of the energy present at one trophic level will not be available for the next one, it is more accurate than the other two pyramids. This is based on a law which states that only about 10% of the energy in a trophic level will go towards creating biomass. In other words, only about 10% of the energy will go into making tissue, such as stems, leaves, muscles, etc. in the next trophic level. The rest is used in respiration, hunting, and other activities, or is lost to the surroundings as heat.

The pyramid of energy is the most widely used type of ecological pyramid, and, unlike the two other types, can never be largest at the apex and smallest at the bottom. It’s an important type of ecological pyramid because it examines the flow of energy in an ecosystem over time.

Ecological pyramid

The diagram below is an example of a productivity pyramid, otherwise called an energy pyramid. The sun has been included in this diagram, as it is the main source of all energy, as well the decomposers, like bacteria and fungi, which can acquire nutrients and energy from all trophic levels by breaking down dead or decaying organisms. As shown, the nutrients then go back into the soil and are taken up by plants.
The loss of energy to the surroundings is also shown in this diagram, and the total energy transfer has been calculated. We start off with the total amount of energy that the primary producers contain,

which is indicated by 100%. As we go up one level, 90% of that energy is used in ways other than to create flesh. What the primary consumers end up with is just 10% of the starting energy, and, 10% of that 10% is lost in the transfer to the next level. That’s 1%, and so on. The predators at the apex, then, will only receive 0.01% of the starting energy! This in-efficiency in the system is the reason why productivity pyramids are always upright.

Recap

Objective type questions

Answer to Objective type questions

Self Assessment Questions

Assignment

Suggested Reading

Reference